The internal-combustion engine, fueled by liquid petroleum products, has long been the mainstay for supplying mechanical power to a broad variety of mobile and stationary machines. There have been many developments for improving the fuel conversion efficiency of such engines. Although most internal-combustion engines which are commercially produced and used today utilize reciprocating pistons which are confined to the motion limits permitted by a crank shaft and interconnected connecting rods, there have also been free-piston internal-combustion engines.
FIG. 1 shows an example of prior art related to free-piston diesel engines which were sold commercially in the 1950s. These engines underwent serious commercial development over three decades. Applications ranged from electric power plants, to ships, to automobiles. Examples of early related to free-piston engines include U.S. Pat. Nos. 1,036,288; 1,571,615; 1,657,641; 2,075,137; 2,595,396; 2,814,552. Among the advantages claimed for these engines were:
Insensitivity to fuel: The free-piston engine automatically adjusts the compression ratio to assure complete combustion of the fuel. PA0 High efficiency: The full-load thermal efficiency of the engine for an automotive application including turbine losses where 32-36%, which was significantly better the spark ignition engines at the time. PA0 Smoothness: The opposed-piston arrangement is inherently balanced with virtually no vibration. PA0 Torque multiplication: The turbine acts to increase torque at low speeds, which may simplify design of the transmission. PA0 Low turbine temperatures: This was a major advantage in the 1950's because of the lack of high-temperature materials; this advantage is of much less importance today with modern materials. PA0 Power-to-weight ratio: Was comparable to that of a conventional gasoline engine with drive train, with further weight reduction possible. PA0 Throttle response: The low mass of the moving parts allows the engine and drive train to respond extremely quickly to increased power requirements.
Despite these major advantages and major investments in development of these engines, there were serious problems that eventually brought an end to serious interest in this technology. These problems included poor part-load efficiency, because the piston operated with essentially a fixed stroke in order to uncover the intake and exhaust ports. In addition the engine had problems with durability of the piston rings and was difficult to start. These practical problems combined with the gradual improvement of competing technologies forced the abandonment of the free-piston internal-combustion engine by the 1960's. Charles Fayette Taylor summarizes the modern view of these engines in his book The Internal-combustion Engine in Theory and Practice, "The results in each case have been unsatisfactory, and the type may now be considered obsolete."
Since then there has been relatively little advancement in free-piston internal-combustion engines. Examples of more recent patents related to these engines include U.S. Pat. Nos. 4,873,822; 5,123,245; 5,363,651; 4,530,317; 4,415,313; 4,665,703; 5,144,917; and 4,205,528. Although most of these engines can be designed and operated to provide good efficiency at a single selected power output load condition, few engines are called upon to operate under only one load condition. Most internal-combustion engines must supply power which varies over a broad range from a low power to a high power. In addition the use of simple sidewall ports for intake and exhaust makes the length of the compression and expansion strokes essentially the same for these engines, which limits their efficiencies. The engines described in the newer patents still have many of the starting, reliability, control, and efficiency problems of the earlier designs and have not been commercialized.
One significant improvement since the 1950s is described in Lenger U.S. Pat. No. 3,772,722. Lenger describes a free-piston engine that uses gas bearings and ceramic components. The principal purpose of these improvements were to eliminate reliability problems associated with piston rings. It also describes the use of ceramic components with a low coefficient of thermal expansion to reduce clearance required at high temperatures. On the other hand, this patent uses simple sidewall ports for intake and exhaust, which limits the cycle efficiency.
The basic objective of the current invention is to produce an engine that retains or improves the desirable features of earlier designs while solving the problems that forced the abandonment of free-piston internal-combustion engines. Fortunately the last 40 years have seen tremendous improvements to the design of free-piston machines for other applications such as Stirling engines and linear compressors. In addition related materials technology and control technologies have greatly improved. The present invention makes use of these improvements in creative ways to solve the problems of the earlier designs.
A unique feature of the current invention is that it allows the expansion stroke to be greater that the compression stroke. The prior art related to free-piston internal-combustion engines does not include this feature. The longer expansion stroke allows a major increase in cycle efficiency. For example, for a simple air cycle, the ideal efficiency increases from 60 to 80% from a conventional Otto cycle to a cycle with full expansion. This advantage represents a 50% reduction in the theoretical losses. Full expansion also greatly reduces the pressure pulses leaving the engine and may eliminate the need for an exhaust muffler. These features give significant cost and performance advantages.
A second unique feature is the uses of gas bearings to support a free piston combined with an expansion ratio that is greater the compression ratio. Gas bearings greatly reduce friction loss, which allows for full use of a longer expansion stroke. Conventional piston rings or other bearings create such high losses that they would negate most of the theoretical advantage of increasing the expansion stroke. Thus our in our invention, gas bearings play an unforeseen role in allowing a major benefit from a longer expansion stroke.
In addition, the use of gas bearings removes the need for oil or other lubricants which eliminates a major problem with high-temperature operation. Modern ceramics and other material allow the engine to be nearly adiabatic, which eliminates the need for a cooling system. Elimination of the lubricant and higher temperature operation can greatly reduce potential emissions from two-stroke engines and facilitates control of the combustion process. Eliminating oil also eliminates the associated maintenance and reliability issues, which is serious problem for two-stroke engines. Hence this setup effectively removes the need for a cooling system and lubrication system, which greatly simplifies the design of the engine.
A third unique feature of this invention is the combination of a free-piston internal-combustion engine that can provide a high-temperature exhaust with a downstream turbine or heat engine. This setup allows the free-piston engine to serve as topping cycle that can give a power-generating system with extraordinarily high efficiency. This topping cycle is applicable to both new and existing power plants. The combination of all these features has the potential to more than double current engine efficiency. These features also decrease emissions and give rapid response to changes in engine load output.
In addition to these advances, the current invention has the capability of providing much more versatile control over a wide range of operating conditions. Three parameters which are important to both the efficiency and the power of an internal-combustion engine are stroke or displacement, expansion ratio, and compression ratio. Conventional crank-type internal-combustion engines permit no controlled adjustment of any of these parameters. The efficiency of an internal-combustion engine is also a function of the ratio of the compression ratio to the expansion ratio. In the conventional internal-combustion engine, neither is variable. The power of an internal-combustion engine is proportional to the mass flow of air, properly mixed with fuel, through the combustion chamber and therefore is also a function of piston displacement. However, piston displacement is not variable in a crank-type engine.
It is a feature and object of some embodiments of the present invention to provide a free-piston internal-combustion engine in which not only are all four of these parameters controllably variable, but additionally the expansion ratio and the compression ratio are adjustable independently of each other. This permits the engine to operate with a different expansion ratio than compression ratio and also allows the displacement or stroke of the engine to be controlled. Consequently, upon a low power demand the engine of the present invention can operate with an expansion ratio which is considerably greater than the compression ratio so that it can operate with more nearly full expansion, resulting in a higher proportion of the heat energy of combustion being converted to mechanical output power. For greater power demands, both the engine displacement and the expansion ratio can be varied so as to achieve maximum efficiency for a given power demand.